Fix for #574 

@rm-openai I'm not sure how to add a test within the repo but I have
pasted a test script below that seems to work

```python
import asyncio
from openai.types.responses import ResponseTextDeltaEvent
from agents import Agent, Runner

async def main():
    agent = Agent(
        name="Joker",
        instructions="You are a helpful assistant.",
    )

    result = Runner.run_streamed(agent, input="Please tell me 5 jokes.")
    num_visible_event = 0
    async for event in result.stream_events():
        if event.type == "raw_response_event" and isinstance(event.data, ResponseTextDeltaEvent):
            print(event.data.delta, end="", flush=True)
            num_visible_event += 1
            print(num_visible_event)
            if num_visible_event == 3:
                result.cancel()


if __name__ == "__main__":
    asyncio.run(main())
````

Now that `ModelSettings` has `Reasoning`, a non-primitive object,
`dataclasses.as_dict()` wont work. It will raise an error when you try
to serialize (e.g. for tracing). This ensures the object is actually
serializable.

Per
https://modelcontextprotocol.io/specification/draft/basic/lifecycle#timeouts

"Implementations SHOULD establish timeouts for all sent requests, to
prevent hung connections and resource exhaustion. When the request has
not received a success or error response within the timeout period, the
sender SHOULD issue a cancellation notification for that request and
stop waiting for a response.

SDKs and other middleware SHOULD allow these timeouts to be configured
on a per-request basis."

I picked 5 seconds since that's the default for SSE

In response to issue #587 , I implemented a solution to first check if
`refusal` and `usage` attributes exist in the `delta` object.

I added a unit test similar to `test_openai_chatcompletions_stream.py`.

Let me know if I should change something.

---------

Co-authored-by: Rohan Mehta <[email protected]>